Stockholders' and Mezzanine equity - Equity Distribution Agreement (Details) - USD ($) $ in Thousands, shares in Millions	3 Months Ended
	Mar. 31, 2017	May 24, 2016
Common Stock
Proceeds from sale of common stock	$ 2,829
Class A common stock
Common Stock
Offering value to sell common stock shares under equity distribution agreement		$ 73,000
Sale of common stock (in shares)	1.2
Proceeds from sale of common stock	$ 2,800
Gross proceeds from sale of common stock	2,900
Professional services expenses	100
Aggregate offering price	$ 67,900